Analysis of the Preliminary Measurements with the Multi-Color Photometric System of the Sky Brightness Monitor of the YNAO

A modern sky brightness monitor (SBM) is a precision instrument used for measuring various atmospheric parameters for an astronomical site during daytime. SBMs are important instruments for site survey programs. The SBM designed in the Yunnan Observatory can be easily dismounted from the main telescope tube by separating the CCD system, which makes it convenient for packing and transportation in the first stage of the site survey in the West China. In order to study possible measurement errors included when assembling the CCD system or in trips, we made a few CCD defocusing experiments during the site survey at Qitai (in Xinjiang) and Shizuishan (in Ningxia) in June 2011. Our statistical analysis shows that even with an obviously defocused CCD, the relative error in measurements is still within 6%. Usually, if the CCD is installed following the line marked on the T-thread nosepiece the relative error should be far less than 6%. Therefore, we think that the error caused by defocusing can be ignored, especially for excellent astronomical sites.